Final answer:
The rhythmic emphasis on beats 2 and 4 on the snare drum is known as a backbeat, essential in creating the energetic and danceable feel of R&B and Rock & Roll.
Step-by-step explanation:
The rhythmic emphasis on beats 2 and 4 on the snare drum in both R&B and Rock & Roll is colloquially called a backbeat. This backbeat is a fundamental part of the rhythm that drives the music and is a key feature that contributes to the energetic and danceable feel of these genres. Both R&B and Rock & Roll share roots in traditional African musical practices that often involve polyrhythms and syncopations, which contribute to the complexity and distinctive sound of these music styles.